This invention relates to signing an electronic document.
An electronic signature is created and verified by means of cryptography. Typically, a private key of the signer of an electronic document is used for creating the electronic signature and a public key of the signer is used for verifying the electronic signature. A recipient of the signed electronic document must have the corresponding public key in order to verify that the electronic signature is the signer's.
In a conventional public-private key implementation, an electronic signature is created by using a hash function to compute a result derived from the signed electronic document and the signer's private key. If the signed electronic document changes, then the hash result will also, with an extremely high probability, change. At the time of signing the electronic document, a hash result is computed and then transformed into an electronic signature using the signer's private key. The resulting electronic signature is statistically unique to both the signed electronic document and the private key used to create it. Typically, an electronic signature is attached to or made part of the electronic document and stored or transmitted with the electronic document.
Verification of the electronic signature is accomplished by computing a new hash result using the same hash function used in creating the electronic signature. The signer's public key is then used to verify whether the electronic signature was created using the signer's corresponding private key and whether the newly computed hash result matches the hash result derived from the electronic signature. If the signer's private key was used and the hash results are identical, then the electronic signature is verified. Verification indicates that the electronic signature was created using the signer's private key and that the electronic document was not altered after it was signed.
The term electronic signature will be used to refer to the code derived from the signed electronic document and a given private key. An electronic signature appearance is a visual manifestation of the signer's signature on the electronic document. An electronic signature appearance can include graphic elements and textual elements. For example, the electronic signature appearance can include an image of the signer's handwritten signature and text indicating the date the electronic document was signed. Because an electronic document cannot be altered after it has been electronically signed without invalidating the electronic signature, the signer cannot alter the electronic signature appearance after signing the electronic document. If the signer is not satisfied with the appearance of the electronic signature on the electronic document, then the signer would have to effectively re-sign another copy of the electronic document, using an electronic signature appearance satisfactory to the signer.